add plugin to be able to use number in permalink (#3240)

Author: hamzaremmal

_plugins/sip_number_permalink.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+# This plugin allows using front matter variables in permalinks.
+# For example: permalink: /sips/:number will use the 'number' front matter field.
+
+Jekyll::Hooks.register :site, :after_init do
+  Jekyll::Drops::UrlDrop.class_eval do
+    def number
+      @obj.data["number"].to_s if @obj.data["number"]
+    end
+  end
+end